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Mansard roof and MWFRS

Mansard roof and MWFRS

Mansard roof and MWFRS

(OP)
How do others handle windloads for a large flat roof structures with a steep slope mansard roof around the perimeter?

Using ASCE7 low-rise building, the steep slope will significantly increase the MWFRS wall pressures compared to a flat roof.  It doesn't make sense that adding a mansard roof above a 3rd or 4th floor should increase the wall pressures at the 1st floor.

RE: Mansard roof and MWFRS

Well you don't say which version of ASCE-7 you are using so I'm going to answer based on 7-98 since I have that edition in front of me.  I would assume a similar answer for more recent editions.

7-98 has 2 figures to address MWFRS pressures, 6-3 and 6-4.  Since you state 'low rise buildings' I'm assuming you are using something similar to figure 6-4 which is for h<60.  From what I've heard, this figure was developed based on requests from the metal building association in order to reduce their loads, notice the table title states walls & Gable roof (and thats what the pictures show) since that is what metal buildings typically look like.  If you have a steep mansard around the perimeter of a multi-story building, I would think Fig 6-4 may not be the best figure to use.

If you use figure 6-3, notice the wall pressure table does not take into account roof pitch or corner zones.  This is a more general table (albeit more conservative for some structures) that can be used for most buildings.

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